The overall goal of the proposed research is to examine the external validity of the DSM-IV subtypes of Attention Deficit Hyperactivity Disorder, which include primarily inattentive (IA), hyperactive/impulsive (HI) and combined (COM) subtypes. Profiles of psychiatric comorbidity and performance on executive function (EF) tasks will be compared among the groups. There is some evidence of differential psychiatric disorders among the groups, but more work needs to be done in this area. It is predicted that the subtypes will have differential psychiatric correlates, with the IA subtype correlating more highly with internalizing disorders and the HI and COM groups correlating more highly with externalizing disorders. Further, very little work has been done on the neuropsychological profiles of the DSM-IV subtypes. It is predicted that the neuropsychological tests that tap inhibition will be differentially impaired in the groups, with only the HI and COM subtypes showing impairment on the variables directly relating to ability to inhibit, such as the commission errors on the Gordon and percent of inhibition and Stop Signal Reaction Time (SSRT) on the Stop Task. Evidence of differential psychiatric profiles and neuropsychological profiles would lend support for the external validity of the DSM-IV subtypes.